Thermoplastic polyesters such as polyethylene terephthalates generally have excellent mechanical properties and chemical properties, so that they are widely used as molded articles such as films and fibers.
However, in processing the polyesters into molded articles, the polyesters have a problem that the production efficiency of the molded articles is low due to the low slipperiness of the polyesters. To overcome this problem, inert particles are conventionally dispersed in the polyester to give irregularities on the surface of the molded articles.
For example, Japanese Laid-open Patent Application (Kokai) No. 52-86471 discloses a method in which inorganic particles having a specific surface area are used, and Japanese Laid-open Patent Application (Kokai) No. 59-171623 discloses a method in which spherical colloidal silica having a particle size of 0.1-1 .mu.m is used. However, since the affinity between the inert particles and the polyesters is generally not good, although these methods are effective for overcoming the problem about the slipperiness, the abrasion resistance and the scratch resistance of the molded articles are not satisfactory.
If the abrasion resistance of a molded article such as a film for magnetic tape is low, powder formed by the abrasion of the film is likely to be generated. As a result, in the process of coating a magnetic layer, spots at which the magnetic coating is not applied are formed, thereby causing drop out. Further, when a magnetic tape is used, since the magnetic tape is made to run while contacting with an apparatus for recording or regeneration, abrasion powder generated by the contact attach to the magnetic layer to cause drop out during recording and regeneration. Still further, generation of a material by abrasion in the calender step in the process of applying the magnetic layer largely decreases the ease of handling of the film in the production of magnetic recording films.
On the other hand, if the scratch resistance of a molded article, for example, a film for magnetic tape is low, scratches are easily formed in the film surface if a foreign matter is generated during the production process of the magnetic tape. As a result, drop out is caused and scratches are easily formed when the tape is made to run at a high speed in use.
Thus, both in the production of the magnetic tape and in use of the magnetic tape, the film for the magnetic tape must have abrasion resistance and scratch resistance.
To overcome this problem, surface treatments of the inert particles have been studied. For example, Japanese Laid-open Patent Application (Kokai) Nos. 63-221158 and 63-280763 disclose modification of the surfaces of colloidal silica particles with glycol group; Japanese Laid-open Patent Application (Kokai) No. 63-312345 discloses modification of the surfaces of colloidal silica particles with a coupling agent; and Japanese Laid-open Patent Application (Kokai) No. 62-235353 discloses surface treatment of calcium carbonate particles with a phosphorus-containing compound. Further, methods in which special particles are employed are proposed in, for example, Japanese Laid-open Patent Application (Kokai) No. 62-172031 (silicone particles) and Japanese Laid-open Patent Application (Kokai) No. 21-29230 (delta type aluminum oxide particles).
However, on the other hand, the conditions in which the films for magnetic tape are used are becomingly severer because higher performance of the magnetic tape and higher production efficiency are demanded. Therefore, the abrasion resistances and the scratch resistances of the films obtained by the above-described various methods are becomingly not sufficient.